1. Field of the Invention
This invention generally relates to a method for manufacturing a stencil mask for patterning, used for manufacture of semi-conductor devices, and more particularly to a method for manufacturing a patterning stencil mask used for a beam process, including photochemical etching or deposition, where beams are introduced for enhancing a reaction of photons, electrons, ions, radicals, and so on, to selectively control the reaction on a particular area of a surface of an object onto which beams are irradiated.
2. Description of the Related Art
In etching or deposition techniques for use in the manufacture of semi-conductor devices, a photochemical reaction process has attracted a great deal of attention. Such a photochemical reaction process is disclosed, for example, in HYOMEN KAGAKU, Vol. 5, No. 4, pp.435, in which a reactive gas and beams (photons) are introduced into a reaction chamber and photons are selectively irradiated onto a certain area of the object (e.g. a surface of a semiconductor substrate) to enhance a reaction. Particularly in photochemical reaction etching, a fluorine series of etching gas is used as a reactive gas, and the reaction caused by this etching gas is enhanced by photons.
In such a photochemical reaction process, no intimate contact type of photoresist mask is required although such a mask is indispensable in photoresist lithography. In other words, the object is directly etched without a photoresist mask, which results in the remarkable advantage of dispensing with a process for forming photoresist mask.
Because of this advantage, the photochemical reaction process has been rated highly, and therefore a stencil mask used for this process is also in great demand, especially in the field of manufacturing of semiconductor devices.
FIG. 10(d) of the accompanying drawings shows a completed prior art stencil mask which is disclosed in Japan Patent Application Serial Number H5-264948. The stencil mask comprises a mask substrate 20, an absorber film 21 formed on the surface of the substrate, and a rear reinforcement film 23 formed on the rear surface of the substrate. A pattern 22 is formed on the absorber film 21. The absorber film 21 has a self-supporting function, in addition to an absorptive or reflective characteristic, for reinforcing a tension (internal stress) of the film itself and preventing itself from being peeled off or loosened.
A window 24 is formed penetrating through the mask substrate to communicate with the pattern 22, through which photons pass. The rear reinforcement film 23 is formed on the rear surface of the mask substrate 20 so as to offset the internal stress generated by the absorber film 21, thereby keeping the mask substrate 20 from being bent. The rear reinforcement film 23 is made of the same material as the absorber film 21, and has the same thickness. However, if the internal stress of the absorber film 21 is very small so that the mask substrate is not warped, the rear reinforcement film 23 is not necessarily required. The rear reinforcement film 23 also serves as an etching mask for forming the window 24 by a back etching technique.
FIGS. 10(a)-10(d) show processes for making the prior art stencil mask for the photochemical reaction process. The conventional method comprises steps of forming an absorber film 21 having a predetermined pattern on the surface of the mask substrate, and forming a window 24 on the mask substrate through which photons pass.
For these processes, a lift-off technique is used for forming the absorber film 21. In a first step, as shown in FIG. 10(a), a photoresist film 25 is formed on a part of the mask substrate 20, which has an inverted mask pattern to the pattern 22, by using photoresist lithography. Then, in a second step shown in FIG. 10(b), an absorber film 21 is deposited on the photoresist film 25 and the exposed surface of the substrate 20 around the photoresist film. In a third step as shown in FIG. 10(c), the photoresist film 25 is removed together with the extra absorber film covering the photoresist film 25 (i.e. lift-off process). As a result, the absorber film 21 remains on the mask substrate with a predetermined pattern. In the next step, the rear reinforcement film 23 is formed on the rear surface of the mask substrate, having an opening corresponding to the window 24. Finally, a window 24 is formed penetrating through the mask substrate with a back etching technique, by using the rear reinforcement film 23 as an etching mask, and the stencil mask is completed.
The thus manufactured stencil mask has an advantage that the mask structure is simplified since the absorber film has a self-supporting function, compared with the conventional stencil mask which is manufactured by separately forming a self-supportive film and an absorber film.
However, the above mentioned stencil mask manufacturing method using a lift-off technique has some problems.
Firstly, the thickness of the absorber film 21 is limited to the thickness t1 of the photoresist film 25. In other words, the reversed-patterned photoresist film 25 must be sufficiently thick to project from the top surface of the absorber film 21 so as to be lifted off. Meanwhile, the absorber film 21 must have sufficient thickness to provide a self-supporting function in addition to the absorptive or reflective function, which makes the thickness t1 of the photoresist film 25 greater. If sufficient thickness is not ensured in the photoresist film 25, the thickness of the absorber film 21 is naturally reduced, which causes either of the absorptive (or reflective) function or the self-supporting function to be lowered. Also, mechanical strength of the stencil mask is reduced and the effective area for forming a pattern 22 is decreased, which hinders the realization of a highly integrated circuit for a semiconductor device.
Secondly, when the thickness t1 of the photoresist film 25 is simply increased to ensure sufficient thickness of the absorber film 21, the width p1 of the mask pattern (see FIG. 10(a)) must be also increased in the photoresist film 25, which results in a reduced precision in forming mask pattern. More particularly, when the photoresist film 25 is exposed (or patterned) with the FIB (Focused Ion Beam) method, as the thickness t1 of the photoresist film 25 increases, the ion-dose amount and the exposing energy must be increased while blooming of the radiation beam profile and decrease of focal depth arise. The photoresist film 25 having a less accurate mask pattern leads to a degraded patterning accuracy in the absorber film 21 formed by a lift-off technique. For this reason, it becomes difficult to form a precise pattern 22 on the stencil mask.
Although these problems has been described in connection with a stencil mask for photo chemical reaction, these are common problems for the general stencil mask used for the manufacture of semi-conductor devices.